It sounds like you want to change your Apple ID. Follow the steps in this link to do so:
Before you change your Apple ID:
- Sign out of all Apple features and services that you use with your current Apple ID.
- Make sure that the email address you want to use for your Apple ID meets these conditions:
- The email address is one that you use regularly, because it will be the primary email address for your Apple ID.
- The email address is not associated with any other Apple ID.
- The email address is not an iCloud email address, which is already an Apple ID. iCloud email addresses end with @icloud.com, @me.com, or @mac.com.
To change your Apple ID, follow these steps:
- Go to My Apple ID (appleid.apple.com), click "Manage your Apple ID", and sign in.
- If you have two-step verification turned on, you'll be asked to send a verification code to the trusted device associated with your Apple ID. If you are unable to receive messages at your trusted device, follow the guidelines for what to do if you can't sign in with two-step verification.
- In the "Apple ID and Primary Email Address section," click Edit.
- Enter the email address you want to use, then click Save Changes. Apple will send a verification email to that address.
- Open the email from Apple, then click Verify Now in the email.
- When the My Apple ID page opens, sign in with your renamed Apple ID.
- If you have two-step verification turned on, you'll be asked to send a verification code to the trusted device associated with your Apple ID.
After you see a message indicating that verification is complete, remember to update all of the stores and services that you use with your Apple ID.
If you're instead talking about changing the signed in email address you're using for the Mail app on your iPhone, check out the steps in here:
Have a great day,